Iron is the fourth most abundant element (˜5%) in the Earth's crust and the least expensive among all transition metals (which is ˜100 times cheaper than minor metals such as cobalt). In its oxide form, iron oxides (e.g., FeO, Fe2O3, Fe3O4) are green materials with little environmental impact and have been investigated as potential anode materials for high-performance lithium-ion batteries, due largely to their attractive specific capacity. Fe2O3 (alpha-Hematite or gamma-Maghemite), for example, carries a theoretical capacity of 1005 mAh/g that is about ˜3 times higher than commercial anode graphite (˜372 mAh/g), and is among the highest in various transition metal oxides (e.g., TiO2, V2O5, Cr2O3, Mn3O4, MoO2, Co3O4, NiO, CuO) (see, for example, FIG. 7). The combined traits of low-cost, nontoxic, corrosion-resistant, and facile synthesis have evidently made Fe2O3 one of the top candidates as anode materials for lithium-ion batteries. Other metal oxides also are of interest.
Unfortunately, some known hurdles need to be overcome before metal oxides can become the components in lithium-ion batteries, including (1) low electrical conductivity of metal oxides (FIG. 7), which curbs the rate performance; (2) defoliation and pulverization of active materials due to the large volume expansion, leading to capacity fading and low cycle life; and (3) scalability, which is desirable for any synthetic approach in order to have practical applications, as the thickness of commercial battery electrodes is typically ˜100-200 μm. Various scientific strategies have hitherto been actively pursued and become a voluminous subject of lithium-ion batteries.
Among various approaches, graphene/metal oxides as anode materials have been under intensive investigations, spurred not only by the high specific capacities of metal oxides (see FIG. 7A), but also by the high electrical conductivity, chemical stability, and mechanical robustness of graphene sheets. To date, a number of composite approaches have been developed, including graphene-anchored, -wrapped, -encapsulated, -layered/sandwiched, or -mixed with metal oxide nanoparticles. Sometimes reduced graphene oxides are used.
Despite high gravimetric energy density and discharge/charge rates often witnessed in some of these composites, most approaches adopt simple dispersion or mixture of graphene with metal oxides, leading to certain shortcomings. First, many strategies only work well when the electrode is very thin. The short diffusion pathway of nanoparticles cannot be taken advantage of when the anode becomes thicker (>100-200 μm for commercial applications), as Li+ has to diffuse through the thickness of the electrode during charge-discharge. This disadvantage inevitably limits commercialization potential. Second, the majority of electrodes are not carbon-black-free or binder-free. Despite the high electrical conductivity of single sheet graphene, carbonaceous species and/or polymeric binders are required in most of these approaches. These extra fillers increase electrode weight but contribute little to the lithium storage, reducing the overall energy density. In addition, carbon additives could cause pseudocapacitive behavior in the low-voltage cycle range that could undermine the role of graphene. Third, the lack of control in microstructure homogeneity and interface structures, which prevents in-depth understanding of graphene/nanoparticle interaction mechanisms. The addition of conductive carbons or polymer binders further clouds such studies.
Because of above reasons, the performance characteristics of many existing graphene/metal oxides cannot easily scale up with the thickness of the electrode. Novel architecture designs are needed in order to solve these and other challenging issues.